A. Field of the Invention
The present invention relates to a load cell used in various lightweight machines or the like, and a method of producing the same.
B. Description of the Related Art
In general, a load cell 1 used in a weighing machine or the like has a strain generating body 6 similar to that shown in FIG. 5. The strain generating body 6 is formed in a hollow rectangular shape by upper and lower beam portions 4, 5 having a fixed rigid body portion 2, a movable rigid body portion 3 and strain generated portions 4a and 5a having small thickness. Strain gauges 7 are adhered to outer surfaces adjacent to the strain generated portions 4a and 5a of this strain generating body 6, respectively. If load is applied to the strain generating body 6, the movable rigid body portion 3 displaces downward relatively to the fixed rigid body portion 2. With the displacement, compression strain or tensile strain occurs on each of the strain generated portion 4a and 5a, and a load acted is measured by detecting the strain occurred by the strain gauges 7.
An electric resistance wire type strain gauge is widely used as the strain gauge 7 of the above-mentioned load cell 1. This electric resistance wire type strain gauge 7 is formed such that a resistance wire 9 made of a metal foil is arranged on a base member 8 made of a synthetic resin in a meandering manner, such as is shown in FIG. 6 of the instant application. Lead wires 10 are connected to the opposite end portions of the resistance wire 9 by, for example, soldering. The strain gauge 7 expands or contracts in Xxe2x80x94X direction in accordance with strain generated on the surface of the strain generated portions 4a and 5a of the strain generating body 6, whereby resistance value of the resistance wire 9 is increased or decreased. Therefore, the magnitude of strain can be detected by monitoring the resistance value.
In the load cell having the above-mentioned structure, it is generally very difficult to prevent a phenomenon known as creep from occurring on the above structure. Creep is a phenomenon in which when a constant load is applied to a strain generating body 6, a measured output value from a strain gauge varies over an elapsed time period starting from the point at which a load is placed. The creep phenomenon is caused for a variety of reasons relating to the elasticity and configuration of the strain generating body, the strain gauge, an adhesive which adheres the strain gauge to the strain generating body, and a coating material included on the strain gauge to protect an electric circuit.
The creep phenomenon has a tendency to reduce the output value over a long period of time in the case where the load cell has a large load applied thereto (relative to the weight capacity rating of the load cell). Further, the creep phenomenon is such that with a small load applied to the load cell (relative to the weight capacity rating of the load cell) the output value increases over a long period of time. In either case, it is an important objective to improve accuracy of the load cell or a weighing machine to suppress this creep phenomenon as much as possible.
Conventionally, the creep characteristic has been partially compensated for by manipulation of a tab ratio defined by the meandering pattern of the resistance wire 9 in a strain gauge. The tab ratio is generally defined as (b/a) where the dimension (b) in Xxe2x80x94X direction of a curved portion of the resistance wire 9 is divided by a width dimension (a) perpendicular to Xxe2x80x94X direction in the resistance wire 9 as shown in FIG. 6. That is, the effects of the creep phenomenon can be at least partially compensated for by specific adjustments of the tab ratio of the resistance wire of the strain gauge, and also making, in combination therewith, particular changes in shape, dimension of the strain generating body such as thickness and the like of the base member in the strain gauge, and other conditions.
However, even through the creep characteristic of the load cell is partially compensated for by manipulation of the tab ratio of the strain gauge as described above, the ability of such manipulation of the tab ratio to compensate for the effects of the creep is extremely limited. As mentioned above, since the creep characteristic differs depending on a rated load, it is impossible to optimize the creep characteristic with respect to the load cell having a wide range of a rated load. Further, there are many different design considerations in making a load cell. The weight capacity rating for each load cell makes it necessary to use a strain gauge having a tab ratio that is specific to the load cell weight capacity. Therefore, many different strain gauges must be manufactured, one strain gauge for each type of load cell. Such a design consideration results in increasing costs for production of such strain gauges.
In particular, since the resistance wire of a strain gauge is formed in a predetermined pattern by etching treatment, if it is necessary to produce various kinds of strain gauges having different tab ratios as mentioned above, it is necessary to prepare many masks used during the etching process. As a result, the characteristics are varied due to uneven etching. Further, even with a finely determined tab ratio, many masks must be prepared for each tab. Therefore, consistency in a desired tab ratio cannot always be obtained with good accuracy since such factors as dispersion of the etching, over-etching or under-etching may occur. It is therefore difficult to appropriately control the creep characteristics of every load cell of each rated load by only control of the tab ratio. Further, it is very difficult to adjust creep characteristics by only control of the tab ratio without changing the gauge length and the pattern shape. In particular, with respect to the load cell of low rated load of a rated load of 6 kg or less, it is almost impossible to make the creep characteristic zero by the above described methods.
Accordingly, one object of the present invention is to provide a load cell in which the effects of creep are essentially minimized if not eliminated. In particular, the object of the present invention is to provide a load cell having low rated load where the effects of creep are substantially zero.
Another object of the present invention is to provide a method of producing a load cell where creep characteristic are substantially reduced or eliminated.
In order to solve the above-mentioned problem, the present invention is characterized by having a strain gauge fixed to a strain generating body. The strain gauge includes a patterned metal foil formed on a base member. The base member is formed from a mixture of a synthetic resin and a small amount of filler and the strain gauge is fixed on a strain generating portion of the strain generating body.
Preferably, the synthetic resin and the filler which form the base member are mixed together so as to define a mixing ratio where the filler is 15 vol % or less of the total volume of the composition forming the base member.
Preferably, the load cell has a rated load of no more than 6 kg includes a strain generating body and a strain gauge.
These and other objects, features, aspects and advantages of the present invention will become more fully apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings where like reference numerals denote corresponding parts throughout.